Printing plate and method for forming the same



Feb.'12, 1935. '.r. c. BRowNE ET AL 1,991,009

' PRINTING PLATE AND METHOD FOR FORMING THE SAME I Filed June 24, 1931 Tizi. l 'A nz f5 INVENTORS Patented Feb. l2, 1935 FORMING THE SAME Theodore C.Brownc, Hinsdale; Ill.,'andV AlfredA C. l

` Buttfield, North Plaintield, N. J.

Application Juneznlesi, serial No. 546.456,' f 12 claims. (cl. 1li-'59) This invention relates to a method and means for forming rubber printing plates and other articles of rubber. a

It has been proposed to form rubber stereo-'- types by vulcanizing rubber into or against the familiar paper stereotype mat and subsequently stripping the rubber from the mat. Fine Work cannot be produced by thisprocess, for it is found that there is asharp limitbelow which satisfactory reproduction of a screen cannot be secured.` Because a'rubber plategives to a letter-press many of thecharacte'ristics of an offset press such as the ability to use less and lighter inks and, print on nearly every surface, there is every reason to produce rubber printing plates bearing as fine a screen or grainas the bes graphic art demands. i

In the present invention, printing platesv are made from rubber which is initiallydispersed in Water, a process which secures great fidelity of reproduction, and, further, Wev dry the .rubber coating as it is formed upon theematrix; which greatly expedites the entire,process and avoids the shrinking, pulling and loss of detail which may occur when the drying operation is carried on as a separate step.

The objects of this invention are to formv jartil i cles of rubber which, even Ain their initial stages of formation have very W Water content; to reducev the manufacturing time previously n ecessary to form and to dry articles made from rubber solutions; to force the uncured stock abstracted from the solution against the matrix or form with considerable pressure; to produce rubber articles which duplicate the form with great fidelity; to produce a tough, dense, `iiexible printingY surface; and to form upon that -surfaceas fine a pattern asmaybe desired..V These and other` objects will become apparent as the specicationproceeds, but it isunderstood that, following the` statutory requirement, We have hereinv set forth the preferred form of our invention and that ,other modications and forms of theinvention may be devised and arranged.

Still other objects and advantages of our, invention will beapparent from the specification. The features ofV novelty which we believe to be characteristic of our invention are set forth with particularity inthe appended claims; Our invention itself', however, will best be understood by reference to the specification and accom- Fig. `2V is' a perspective View of printing plate, and ,a

Fig. 3 isVa 'partial sectional view -illustrating one ofthe early stepsin the process of producing matrix in accordance with our invention. l j Whileritve. shall vpresent this invention as a the completed `method'cf manufacturing printing plates,-the

process "is suitable to producearticles of rubber in almostlanyfshape, even in large, flat'sheets. Thespecic-naturof the article to be produced governa-of course, and the slight variations in the procedureVv or apparatus made necessary by the'desired product will become evident asthe processv is understood vandas this specification proceeds.; f A 0 Anabsorbent mould or form 11 is made of Whatever is -tobe reproduced., To make a printing plate, ytype locked in a chase, a photoenf gravers plate, or other material which is com-f` monly received by an electrotyper and Whichwe i generally designate byV the Word form 1716, is covered with liquid plaster.v

An imperfect mould can only" mean an imperfect result, and, although We have produced satisfactory matrices and moulds by following certain `Well known plaster casting procedurea the results `are too' erratic. Pits i in the surface `caused'by air `bubbles frequently appear.

We avoid this `dif'culty by 'mixing theA powdered plasterV with water-"from `which muchl of the-v vdissolvede'gas has beenjremoved and then Vstirring `theY two together under a Asubstantial vacuum.'v The air, trappedbetween the grains of povvder, foams outQACasting is safe when all foaming has ceased and the plaster has settled back toA a thick, viscous liquid; stirring expedites the process;

Stickingofv the plaster to the form isV prevented by a parting medium. As will laterappear, the surface condition of the matrixA is'of `great im.- portance, jporisequently,Y an ill-chosen parting mediummay causelseriousY trouble. Nothing in e the nature `of animpervious membrane4 nor a Water repellent substance should be used.` A mild,r neutral soap, however,` We "nd satisfactory.` fIt neitherl interfereswith the absorption phenomena later relied upon, nor doesit cause the latex to precipitate. We brush'a` 10% solution carefully. over the form before the plaster is applied. f

Commercial plasters vary greatly.v We suggest, therefore, that the mutual proportions of water and` plaster be determined by test. for each lot of plaster used.,f The desideratum isto .produce H a vmould which, l when hardened dried, will absorb all the Water possible. Me-

' chanical strength and density may be sacrificed,

in part, to secure this end.

In casting printing plate matrices where a very perfect impression is necessary, it is even desirableto place the form on a circuit 15 in a' vacuum box 17 and apply the plaster while the pressure isreduced. This is indicated in Fig. 3 in which 20 represents the hopperY containing the plaster 22 ready to be cast, which flows Onto the form'through the conduit 18, Which is provided plaster vis sufficiently dry. It may then be left to air dry; but we prefer to expedite the process and to dry the cast thoroughly at a mildheat.

- f We shall use the term latex hereafter to denote a water dispersion of rubber (either the in rubber compounds.

natural latex or artificial dispersions) and dispersions of other natural or artificial substances which behave inY a similar manner and which -may be coagulated or condensed to form a dense,

flexible substance.'V We also use the word solution, in the'popular, not the scientific, sense and V,intend Vthat water suspensions as well as true solutions shall be described by this general term.V A solution ofV latex is made up containing a curing agent, an eccelerator, and the common loading and protective substances usually found The particular formula isl ofimportance only with regard tothe use to whichA the rubber product will be put. If a printing plate isto be used with a greasy ink, a formularwhich will produce a grease resistant rubber should be chosen, but wide variations in the formulae have, substantially, no effect upon the process itself. f Y,

The plaster matrix Vis placed face-up on a table and a small dam placed around its edges. A scoopful of compounded latex is then poured over its surface `,The plaster seizes the Water from the latex and a tough, dense coat of oompound is rapidly built on its surface. It will be obvious thatrany specific gures depend upon equallyspecic conditions viz. the type ofv the plaster, the dryness of the matrix, the kind of latex, etc., but we have secured coagulated coats of compound about one millimeter thick in ve y minutes.

Whenfsufacientfrubber has been built up upon the matrix, Vthe remaining latex fis poured off. .The vmatrix couldbe dipped ina tank of latex, if desired. We prefer the process. outlined because it obviates the necessity of waterpro'ong orprotecting the rear surface and the sidewalls of. the plaster.VY In a minute or two afterV the latex has been poured olf, the characteristic appearance of alatex-wet surface is gone. Coaguv lation ofthe entire lm has taken place. Subsequently Water is expressed as syneresis takes place. The matrix with the layer oflatex thereon will then have the appearance as diagrammatically indicated in Fig. 1, 11 being the form and'12the layer of coagulated latex.

, The process will come to an end when the matrix is saturated, but if its back be sub-V f `jected to a substantial vacuum and the water A25 indicates Yany suitableV vapor be removed by freezing or otherwise, saturation of the matrix Vcan be prevented.

It is evident that the forces due to the absorption are high and full advantage is taken of them, for the rubber is driven into Zand du-V plicates (in reverse) the finest markingsrin the mould with the utmost delity.

Usually, and preferably, we compound a rubber accelerator of the ultra Vtype with the latex because it makes the following procedure possible. A piece of properly compounded backber plate. The matrix, stock, and plate are then clamped in a vulcanizingpress which isY set to form a printing plate .of theV requisite may Vuse sulphur chloride or some equivalent V cold-cure.

After curing', the rubber plate is Vstripped from Y the matrix which is thoroughly `,dried and is then ready for use again. i

The plate ,may be aiTixed to any approved block in any suitable manner, vas indicated inV Fig. 2, in which 12 is the printing surface of the plate, 13 the backing stock-and 14 the mountingjblock. Wooden blocks as used with electrotypes are reasonably satisfactory, butra preferable mounting is upon a metal block. -This may be an adaptation of the metal base now commonly available for electrotypes, electroing stockrl3 is laid over the newly formed rubplated however, with'a suitable metalto make t bonding of the rubberplate possible -Theordif nary basing vmetal alloy will bond to rubber only with great diniculty. Y We may cure the latex, backing stock and bonding cement all in one operation. In that case, the electroplated face of the metal base covered with afbon-ding-to-metal type cement is Ylaid upon the backing stock and the vulcanizing press is set to produce a type-high plate after -the cure. f f For the nest work, some correction in the printing surface level may be required. We do not apply the base initially in this case, but proof the plate tlrst.v If parts are too high, they are corrected by grinding down the back of the plate, or if too low, they may be built up by thin shims of rubber or dabs of cement.

Variants ofthe process may be as follows:

A iiong or" `paper is prepared fand pressed over the form in the usual manner. ilong's'ras are now used in stereotype work are usable, butV we have found it desirable to omitY allclay and loaders from the paste and to substitute a ne, absorbent tissue for the Italian tissue usually placed upon the face of the ong. I'he n absorption of thelatter ilong is much better.

When the impression has been made and dried according to the practice in stereotype plants, We pour compoundedlatex over'the face of the mat. The procedure now follows that set out above except that the mat is laid upon a bed of quick'setting cement to form a flat bearing to receive the pressureV of the vulcanizing press.

The matrix may also be formed of fine wood iour or other pulverized absorbent matter. Wood `flour, preferablypassing 200 mesh, is mixed with a binding agent. If a waterproof binder be used, the mass should be damp only, not saturated With the binder, for it is advantageous to leave microscopic voids and absorption channels. Ordinary non-Waterproof adhesives may be used. i

The form is covered with some material such as graphite, talc, or soft soap to prevent sticking and the damp mass is forced into the form by an electrotypers press or its equivalent. When the matrix is dried, it is covered with latex as before.

We have foundit advantageous, when using certain matrices, to incorporate into the material of the matrix one or more of the recognized precipitating or coagulating agents for latex such as sodium acetate.` Penetration of @the rubber `into the matrix itself is prevented bythis expedient which also inay'increase the Vplaster and covering the impression with latex,

whereby a coating of rubber is formed upon the plaster surface. 1. .Y y

2. VThat process of i forming printing `plates which comprises forming a matrix of al Water absorbent material and covering that portion of the matrix which bears the impression with compounded latex, whereby Water 'is extracted from the latex by the matrix and a coating of rubber Vis formed thereon.

Yplate which `includes forming an absorbent ma- `3. The process of manufacturing a printing trix for the plate, placing compounded latex in contact with the matrix, and subsequently cur-A ing the rubber deposited from the latex at a temperature below 212 F.

4. The process of manufacturing a rubber printing plate which includes extracting Water from the compounded latex by means of the matrix; whereby a layer of rubber is built up upon the surface thereof, and subsequently curing the rubber so depositedwhile it is still in contact With the matrix.

5. That process of forming printing platesk which includes forming a rmatrix ofV Water absorbent material and covering that portion of the matrix which bears the impression withV `together to form a printing plate.

6. In a process for forming rubberv printing plates, thesteps Whichinclude forming a WaterV absorptive mould having a coagulating agent `for latex incorporated thereinand then collect-` ing and coagulating uponV the moulda layer of rubber by exposing the surface of the mould to a bath of compounded latex.

'7. That process of forming a rubber printing plate which includes extracting liquid from latexinto an absorptive matrix whereby a layer of rubber is formed thereon, and then curingthe rubber in place upon the matrix under conditions of temperature and pressure which Will prevent the formation of steam.

8. A process` for forming a rubber printing plate which includes mixing a moulding composition and a liquid and treating under a vacuum to remove the entrapped air, forming an impression of the article to be duplicated in the moulding composition, and later collecting -rubber from latex upon the surface of the mould so formed. Y

9. A printing plate comprising a facing of coagulated and then materially dehydrated latex rubber, a backing layer of sheet rubber, and .a supporting base all so joined together as to form ra unitary printing element.

10. A printing plate having its `printing surface and a layer of material immediately adjacent thereto deposited in its nal configuration from an aqueous dispersion of rubber Which Y layerY has been materially dehydrated during the process of its formation. i

11. A printing plate having a Working Vsurface formed in place `from a water dispersion of rubber, an intermediate layer of rubber, and an element providing substantial mechanical support.

12. The process of forming relief-bearingV THEODORE C. BROWNE. ALFRED o. BUTTFIELD. 

